Operational amplifiers have been known for many years and serve many varied needs in circuit designs. For example, operational amplifiers are used in many battery operated circuits. To extend the battery life of such battery operated circuits, the battery operated circuits are being designed to operate from lower supply voltages. As is known, by reducing the supply voltage, the battery life of a battery operated circuit can be extended.
As the drive for extended battery life pushes supply voltages lower and lower, dynamic output range of operational amplifiers becomes a critical design issue. As is known, the output of an operational amplifier has a dynamic range that is less than rail to rail, i.e., from the supply voltage to the return voltage. With a dynamic output range less than the rail to rail capabilities, the output of an amplifier may be insufficient to drive a subsequent stage in the circuit. For example, if the supply voltage is 2.7 Volts, a typical dynamic output range will be 0.1 Volts to 2.1 Volts. (Various output ranges, and the masons therefor, will be discussed below with reference to prior an FIGS. 1-3.) When the operational amplifier is supplying its output to a digital circuit, the 2.1 Volts, in many applications, is insufficient to provide a logic "1", where a voltage of 2.5 Volts is typically needed to guarantee a logic "1".
FIGS. 1-3 illustrate various operational amplifiers and their corresponding dynamic output range. FIG. 1 illustrates a basic operational amplifier which has been known in the art for sometime and provides an amplification of the difference between a first input signal and a second input signal. The dynamic range of the output, however, ranges only from the reference ground potential, zero for this particular example, to a voltage level of (V.sub.DD -(V.sub.GS +V.sub.DS)). Thus, the prior art operational amplifier could not pull its output up to the supply voltage V.sub.DD, but only up to the supply voltage minus the gate to source voltage of one of the field effect transistor (0.6 Volts) plus the drain to source voltage of a field effect transistor in the current source (0.2 Volts). For a 2.7 Volt supply, the maximum output voltage of the operational amplifier is 1.9 Volts (2.7-(0.6 +0.2)).
FIG. 2 illustrates another prior art operational amplifier that has an improved dynamic range over the operational amplifier of FIG. 1. This embodiment includes a basic operational amplifier and an additional circuit coupled to the output of the operational amplifier. The additional circuit allows the output voltage to be pulled down to a reference ground potential of zero and up to V.sub.DD -V.sub.GS. Thus, the additional circuit improves the dynamic range of the amplifier of FIG. 1 by eliminating the V.sub.DS term, however, the gate to source voltage V.sub.GS limits the upper voltage level of the output. For a 2.7 Volt supply, the maximum output voltage of the operational amplifier is 2.1 Volts (2.7-0.6).
FIG. 3 illustrates still another prior art operational amplifier. The configuration of FIG. 3 includes a basic operational amplifier having its output coupled to an additional circuit. The additional circuit included a current source and an n-channel field effect transistor (FET). While the output of the amplifier can be pulled up to (V.sub.DD -V.sub.DS), a higher level than either the output of the amplifiers shown in FIG. 1 and FIG. 2, the output can only be pulled down to a level of V.sub.DS because the n-channel FET. For a 2.7 Volt supply, the output range of the operational amplifier is 0.2 Volts to 2.5 Volts.
Thus, these prior art amplifiers fail to provide a sufficiently wide dynamic range that could be used in low voltage circuitry. Therefore, a need exists for an operational amplifier that has a wide dynamic range and that may be used in low voltage level applications.